TSH vs Free T4: What Each Tells You

TSH and free T4 are the two blood tests most often used to assess thyroid function, and they answer different questions. One reflects the signal the brain sends to the thyroid, and the other reflects what the thyroid is actually producing. Reading them together is what makes thyroid testing informative.

What each test measures

TSH (thyroid-stimulating hormone) is made by the pituitary gland, not the thyroid. It is the pituitary's instruction to the thyroid: when thyroid hormone levels seem low, the pituitary raises TSH to push the gland to produce more; when levels seem high, it lowers TSH. So TSH is best thought of as a sensitive readout of how the body perceives its thyroid hormone supply.

Free T4 (free thyroxine) measures the active, available fraction of thyroxine in the blood — the thyroid's main hormone output that is not bound to carrier proteins. It reflects what the thyroid is actually releasing into circulation. Together, the pituitary's signal (TSH) and the gland's output (free T4) give a picture of both sides of the feedback loop that keeps thyroid hormone in balance.

How they differ

The key difference is the source and the meaning. TSH comes from the pituitary and is an indirect but very sensitive indicator: small changes in thyroid hormone can produce noticeable shifts in TSH, which is why it is often the first test. Free T4 comes from the thyroid and is a more direct measure of hormone output. Because of this, TSH tends to flag a problem early, while free T4 helps show how large the change in actual hormone is and where the issue may lie.

The two also move in opposite directions in common patterns. In a typically underactive thyroid, TSH rises while free T4 falls, because the pituitary is pushing harder against a sluggish gland. In a typically overactive thyroid, TSH falls while free T4 rises. When the two do not fit these expected pairings, that mismatch is itself informative and may prompt a clinician to look further, for example at the pituitary rather than the thyroid.

Side-by-side comparison

The table below summarises typical differences. Any laboratory values referenced are illustrative only and vary by laboratory, age, sex, and the assay used.

When each matters

TSH matters most as a sensitive screening test. Because the pituitary reacts strongly to small changes in thyroid hormone, TSH can reveal an early or subtle shift in thyroid function, which is why it is frequently checked first. Free T4 matters when more detail is needed: it shows the size and direction of the change in actual hormone, confirms what the TSH is suggesting, and is especially helpful when the two results need to be compared.

There are situations where TSH alone can mislead, such as certain pituitary conditions, recent changes in thyroid status, some medicines, and pregnancy, where the expected relationship between the two may shift. In these cases free T4 adds important context. A clinician chooses which tests to run and interprets them together with symptoms and history.

How they relate

TSH and free T4 are two ends of the same feedback loop. The pituitary releases TSH to drive the thyroid, the thyroid releases T4 in response, and the level of free T4 in turn feeds back to adjust how much TSH the pituitary makes. In a balanced system, the two settle into a stable relationship; when something disturbs the thyroid, they typically move in opposite directions, which is what makes the pair so useful. Reading them together — rather than either alone — lets a clinician see whether a change originates in the thyroid, in the pituitary signal, or elsewhere.

For related comparisons, see T3 vs T4 and Hypothyroidism vs Hyperthyroidism. You can also browse the blood tests and hormones sections.